It is known to provide steering for a powered wheelchair by varying either the power or the direction of rotation to either of the fixed-axle driving wheels whether they are at the front or the rear of the vehicle. The remaining wheels or pair of wheels are castors.
When such a wheelchair crosses a slope the weight of the chair and its occupant turns the castors in the direction of the slope causing the chair to be deflected from the selected direction. Furthermore, when a wheel on one side of the chair meets an obstruction or resistance greater than the other side there is a tendency for the chair to turn in the direction of that obstruction. If the slope, obstruction or resistance is considerable, then the unwanted change of direction can seriously endanger the occupant. It is for this reason that powered wheelchairs with castor wheels are generally considered unsafe except when used on comparatively flat and even surfaces.
It is also known that to overcome this problem a steering motor can be added to the castors and arranged to be controlled by a joystick, but this arrangement provides only a limited steering radius which, although perfectly acceptable for outdoor use, is not acceptable indoors.